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United States Patent |
5,179,678
|
Kawamura
|
January 12, 1993
|
Address/control signal input circuit for a cache controller which clamps
the address/control signals to predetermined logic level clamp signal
is received
Abstract
A cache controller is coupled to a cache memory coupled through a bus to a
microprocessor, for controlling the cache memory in response to a signal
from the microprocessor. The cache controller comprises an address signal
terminal and a control signal terminal for receiving an address signal and
a control signal from the microprocessor, and a clamp signal terminal for
receiving a clamp signal from the microprocessor. The cache controller
also includes an input circuit coupled to the address signal terminal, the
control signal terminal and the clamp signal terminal for outputting the
address signal and the control signal as they are when the clamp signal is
inactive and for generating an output address signal and an ouput control
signal having a predetermined logic level when the clamp signal is active.
Inventors:
|
Kawamura; Eiji (Tokyo, JP)
|
Assignee:
|
NEC Corporation (Tokyo, JP)
|
Appl. No.:
|
637962 |
Filed:
|
January 9, 1991 |
Foreign Application Priority Data
| Sep 08, 1987[JP] | 62-226015 |
Current U.S. Class: |
711/118; 710/22 |
Intern'l Class: |
G06F 013/14 |
Field of Search: |
395/425
|
References Cited
U.S. Patent Documents
4090236 | May., 1978 | Bennett | 364/200.
|
4112490 | Sep., 1978 | Pohlman | 364/200.
|
4245307 | Jan., 1981 | Kapeghian | 364/200.
|
4458313 | Jul., 1984 | Suzuki | 364/200.
|
4575792 | Mar., 1986 | Keeley | 364/200.
|
4602327 | Jul., 1986 | La Violette | 364/200.
|
4720811 | Jan., 1988 | Yamaguchi et al. | 364/900.
|
4809167 | Feb., 1989 | Pawloski | 364/200.
|
4819158 | Apr., 1989 | Miyashita | 364/200.
|
4835678 | May., 1989 | Kofuji | 364/200.
|
4860244 | Aug., 1989 | Bruckert | 364/900.
|
4888689 | Dec., 1989 | Taylor | 364/900.
|
Primary Examiner: Anderson; Lawrence E.
Assistant Examiner: Coleman; Eric
Attorney, Agent or Firm: Foley & Lardner
Parent Case Text
This application is a continuation of application Ser. No. 07/242,140,
filed Sept. 8, 1988, now abandoned.
Claims
I claim:
1. A system comprising:
a microprocessor and a DMA controller which are coupled to a data bus, an
address bus and a control bus, the DMA controller generating a HOLD
request signal which is supplied to the microprocessor, and the
microprocessor responding to the HOLD request signal by outputting a HOLD
acknowledge signal to the DMA controller and by bringing terminals of the
microprocessor connected to the data bus, to the address bus, and to the
control bus into a high impedance condition,
a cache data memory coupled to the data bus,
a cache controller having an address signal input terminal coupled to the
address bus for receiving an address signal from the microprocessor and a
control signal input terminal coupled to the control bus so as to receive
CPU status signals contained in a control signal outputted by the
microprocessor onto the control bus, and
a clamp signal generator coupled to receive the HOLD request signal from
the DMA controller and the CPU status signals on the control bus, the
clamp signal generator generating a clamp signal and outputting the clamp
signal to the cache controller if the HOLD request signal becomes active
when the CPU status signals indicate a predetermined condition, the cache
controller further including a clamp signal input terminal coupled to
receive the clamp signal from the clamp signal generator, a first signal
input circuit coupled to the address signal input terminal to produce an
internal address signal, a second signal input circuit coupled to the
control signal input terminal to produce an internal control signal, and
means coupled to the clamp signal input terminal and to the first and
second input circuits for causing the first and second input circuits to
respond to the address signal and to the CPU status signals to produce the
internal address signal and the internal control signal, respectively,
when the clamp signal is inactive, and for causing the first and second
input circuits to clamp the internal address signal and the internal
control signal at a predetermined logic level, respectively, irrespective
of potential levels at the address signal input terminal and at the
control signal input terminal, when the clamp signal is active.
2. A cache controller coupled to a cache memory for controlling the cache
memory in response to a signal from a microprocessor, the cache controller
comprising:
an address signal input terminal and a control signal input terminal
coupling through a bus to the microprocessor to receive an address signal
and a control signal from the microprocessor;
a clamp signal input terminal coupled to receive from the microprocessor a
clamp signal which is rendered active when the microprocessor brings, into
a high impedance condition, terminals of the microprocessor for outputting
the address signal and the control signal, the bus being subjected to take
an intermediate level between one logic level and another logic level by
the microprocessor bringing the terminals of the microprocessor into the
high impedance condition;
a signal input circuit, coupled to the address signal input terminal and to
the control signal input terminal, to receive signals on the bus to
produce an internal address signal and an internal control signal; and
means, coupled to the clamp signal input terminal and to the signal input
circuit, for causing the signal input circuit to respond to the address
signal and the control signal from the microprocessor to produce the
internal address signal and the internal control signal, when the clamp
signal is inactive, and for causing the signal input circuit to clamp the
internal address signal and the internal control signal at a predetermined
logic level irrespective of the intermediate level of the bus, when the
clamp signal is active.
3. A cache controller as claimed in claim 2 wherein the input means
includes a first AND gate having a first non-inverted input connected to
the address signal input terminal, a second inverted input connected to
the clamp signal input terminal, and an output connected to an address
output terminal, and a second AND gate having a first non-inverted input
connected to the control signal input terminal, a second inverted input
connected to the clamp signal input terminal, and an output connected to a
control signal output terminal.
4. A cache controller as claimed in claim 3 wherein each of the first and
second AND gates includes at least one AND circuit which includes a first
input connected to receive one bit of the signal supplied to one of the
address signal input terminal and the control signal input terminal, a
second input connected to receive an inverted signal of the clamp signal
supplied to the clamp signal input terminal, a pair of p-channel
transistors having their respective gates connected to the first input and
the second input and also having commonly connected sources connected to a
supply voltage, the p-channel transistors being commonly connected at
their drains, a pair of series-connected n-channel transistors connected
between ground and the commonly connected drains of the p-channel
transistors, the n-channel transistors being connected at their gates to
the first and second inputs, respectively, and an inverter having an input
connected to the commonly connected drains of the p-channel transistors.
5. A cache controller as claimed in claim 3 further including a plurality
of clamp signal input terminals connected to a logic circuit for
generating a resultant clamp signal which is supplied to the second
inverted input of each of the first and second AND gates.
6. A cache controller as claimed in claim 2 further including a plurality
of clamp signal input terminals connected to a logic circuit for
generating a resultant clamp signal which is supplied to the input means.
7. A cache controller as claimed in claim 1 wherein the cache controller
includes a first AND gate having a first non-inverted input connected to
receive the address signal, a second inverted input connected to receive
the clamp signal, and an output generating an address signal, and a second
AND gate having a first non-inverted input connected to receive the CPU
status signal, a second inverted input connected to receive the clamp
signal, and an output generating a control signal.
8. A cache controller coupled to a cache memory for controlling the cache
memory in reponse to a signal from a microprocessor, the cache controller
comprising:
an address signal input terminal coupled to receive an address signal from
the microprocessor;
a clamp signal input terminal coupled to the microprocessor to receive a
clamp signal which is rendered active when the microprocessor brings, into
a high impedance condition, a terminal of the microprocessor for
outputting the address signal;
an address input circuit, coupled to the address signal input terminal, to
produce an internal address signal; and
means, coupled to the clamp signal input terminal and to the address input
circuit, for causing the address input circuit to respond to the address
signal from the microprocessor to produce the internal address signal,
which changes between logic levels in response to a change in a logic
level of the address signal, when the clamp signal is inactive, and for
causing the address input circuit to clamp the internal address signal at
a predetermined logic level irrespective of a potential level at the
address signal input terminal, when the clamp signal is active.
9. A cache controller as claimed in claim 8 wherein the input means
includes an AND gate having a first non-inverted input connected to the
address input terminal, a second inverted input connected to the clamp
signal input terminal, and an output connected to an address output
terminal.
10. A cache controller as claimed in claim 9 wherein the AND gate includes
at least one AND circuit which includes a first input connected to receive
one bit of the signal supplied to the address input terminal, a second
input connected to receive an inverted signal of the clamp signal supplied
to the clamp signal input terminal, a pair of p-channel transistors having
their respective gates connected to the first input and the second input
and also having commonly connected sources connected to a supply voltage,
the p-channel transistors being commonly connected at their drains, a pair
of series-connected n-channel transistors connected between ground and the
commonly connected drains of the p-channel transistors, the n-channel
transistors being connected at their gates to the first and second inputs,
respectively, and an inverter having an input connected to the commonly
connected drains of the p-channel transistors.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a microprocessor, and more specifically to
a controller for a cache memory incorporated in the microprocessor.
2. Description of Related Art
At present, most microcomputers have a high speed memory called a "cache
memory" (simply called a "cache" hereinafter) in addition to the usual
(external) main memory and secondary memories. In general, the access
speed to the main memory is very slow, for example a few tenths to a few
hundredths of the internal processing speed of a microprocessor or central
processing unit (CPU), and therefore, if the microprocessor fetches
necessary programs and data from the main memory at each execution of an
instruction, the operation speed of the microprocessor is limited by the
access speed of the main memory. In other words, high speed processing
cannot be expected. For solving this problem, the cache has been proposed
and is widely used at present. The cache copies a portion of the programs
and data that are stored in the main memory and that are most heavily
used, and in the operation of the microcomputer, the microprocessor
accesses the cache (i.e., high speed access memory) internally provided in
the microcomputer in place of accessing the external main memory, so that
the microprocessor can execute the operation at a high speed.
In a cache associated to a microprocessor, hitherto, an address signal from
the microprocessor is preferably directly supplied to the cache in order
to make as short as possible a time from the moment the address signal is
outputted from the microprocessor to the moment there is discriminated
whether or not the cache is hit. However, some microprocessors are adapted
such that when the microprocessor is in a RESET condition, in an INTERRUPT
condition or in a BUS HOLD condition, output terminals of the
microprocessor are put in a high impedance condition. If the output
signals of this type of microprocessor are directly coupled to the cache
as mentioned above, an intermediate level propagation will occur, so that
inputs of the cache become logically unstable.
In order to overcome the above drawback, it has been proposed to pull up
the signal lines from the microprocessor to the cache, so that when an
input signal will not been put in the high impedance condition. However,
pull-up resistors and their associated connection lines for the pull-up
purpose will inevitably induce a floating or parasitic capacitance, which
will cause a delay in signal propagation. This delay will often give rise
to a significant adverse influence on the timing of an operation of cache
controller.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a cache
controller which has overcome the above mentioned defect of the
conventional one.
Another object of the present invention is to provide a cache controller
capable of logically stabilizing the input signal without pulling up the
input signal line.
Still another object of the present invention is to provide a cache
controller capable of logically stabilizing the input signal without
pulling up the input signal line and capable of minimizing the delay of
the input signal propagation.
The above and other objects of the present invention are achieved in
accordance with the present invention by a cache controller coupled to a
cache memory coupled through a bus to a microprocessor for controlling the
cache memory in response to a signal from the microprocessor, comprising
an address signal terminal and a control signal terminal for receiving an
address signal and a control signal from the microprocessor, a clamp
signal terminal for receiving from the microprocessor a clamp signal which
is rendered active when the microprocessor brings, into a high impedance
condition, terminals of the microprocessor for outputting the address
signal and the control signal, and input means coupled to the address
signal terminal, the control signal terminal and the clamp signal terminal
for outputting the address signal and the control signal as they are when
the clamp signal is inactive and for generating an internal address signal
and an internal control signal having a predetermined logic level when the
clamp signal is active.
With the above mentioned arrangement, if the clamp signal is active, the
output address signal and the output control signal having the
predetermined logic level are generated. Therefore, when the address
signal and the control signal outputted from the microprocessor are
brought into a high impedance condition, the clamp signal is rendered
active, and accordingly, the adverse influence of the high impedance will
be prevented. In addition, since there is no pull-up circuit for the
address signal and the control signal, a delay of these signals inputted
to the cache memory is minimized.
The above and other objects, features and advantages of the present
invention will be apparent from the following description of preferred
embodiments of the invention with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of one example of a conventional cache
controller;
FIG. 2 is a block diagram illustrating a basic construction of the cache
controller in accordance with the present invention;
FIG. 3 is a block diagram of a first embodiment of the input circuit of the
cache controller embodying the present invention;
FIG. 4 is a circuit diagram of an AND gate used in the input circuit of the
cache controller shown in FIG. 3;
FIG. 5 is a block diagram of a second embodiment of the input circuit of
the cache controller embodying the present invention; and
FIG. 6 is a block diagram of another embodiment of the cache controller
embodying the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, there is shown a diagram of one example of a
conventional cache controller. The shown system includes a CPU 10, and a
cache controller 12 coupled to the CPU 10 through an address bus 14 and a
control bus 16 so that the cache controller 12 receives an address signal
and a control signal from the CPU 10. In addition, the address bus 14 and
the control bus 16 are pulled up through resistors 18 and 20 to a supply
voltage Vdd, respectively.
Thus, the address signal and the control signal outputted from the CPU 10
are pulled up through the resistors 18 and 20, and then inputted to the
cache controller 12. Therefore, for example, when the address signal
outputted from the CPU 10 is at a logically low level, the logically low
level address signal is inputted to the cache controller 12. When the
address signal outputted from the CPU 10 is at a logically high level, the
logically high level address signal is inputted to the cache controller
12. In addition, when the address signal outputted from the CPU 10 is in a
high impedance condition, a logically high level address signal is then
inputted to the cache controller 12. Namely, the cache controller 12 will
not directly receive the address signal in a high impedance condition.
In the system shown in FIG. 1, however, since the address bus 14 and the
control bus 16 are pulled up through the resistors 18 and 20, floating or
parasitic capacitances attributable to the pull-up resistors 18 and 20 and
their associated connection lines for pull-up will cause a delay in signal
propagation. Namely, the time of signal propagation from the moment the
address signal or the control signal is actually outputted from the CPU 10
to the moment the address signal or the control signal is actually
inputted to the cache controller 12 will become longer than a case in
which the address signal and the control signal are not pulled up.
On the other hand, the cache controller 12 should be made to minimize the
time from when the address signal is inputted to the moment a hit signal
is outputted. Accordingly, the address signal should be inputted to the
cache controller as soon as possible. For this reason, the cache
controller is constructed to operate at a high speed.
However, the delay caused by the floating or parasitic capacitances
attributable to the pull-up resistors 18 and 20 and their associated
connection lines will give a significant adverse influence on the
operation of the cache controller, particularly on a critical pass on the
timing.
Referring to FIG. 2, there is shown a block diagram illustrating a basic
construction of the cache controller in accordance with the present
invention.
The shown system includes a CPU 30, and a cache controller 32 coupled to an
address bus 34 and a control bus 36 which are also coupled to the CPU 30
so that the cache controller 32 receives an address signal and a control
signal directly from the CPU 30 through no intermediary of a pull-up
circuit. In addition, the cache controller 32 is made to receive a clamp
signal through a clamp signal line 38 from the CPU 30.
Turning to FIG. 3, there is shown a block diagram of a first embodiment of
an input circuit of the cache controller shown in FIG. 2.
The shown input circuit is provided as an internal circuit of the cache
controller 32 shown in FIG. 2 and includes three input terminals as
external terminals of the cache controller 32. A first input terminal is
an address input temrinal 40 coupled to the address bus 34 for receiving
the address signal from the CPU 30. A second terminal is a control signal
input terminal 42 coupled to the control bus 36 for receiving the control
signal from the CPU 30. A third terminal is a clamp signal input terminal
44 coupled to the clamp signal line 38 for receiving the clamp signal from
the CPU 30.
The input circuit also includes a first AND gate 46 having a first,
non-inverted, input connected to the address input terminal 40 and a
second, inverted, input connected to the clamp signal terminal 44. This
first AND gate 46 has an output connected to an address output terminal 48
that is provided as an internal terminal of the cache controller 32. In
addition, a second AND gate 50 is connected at its first, non-inverted,
input to the control signal input terminal 42 and at its second, inverted,
input to the clamp signal terminal 44. This second AND gate 50 is also
connected at its output to a control signal output terminal 52 that is
provided as an internal terminal of the cache controller 32.
For simplification of the drawing, the address input terminal 40, the
control signal input terminal 42, the AND gates 46 and 50, the address
output terminal 48 and the control signal output terminal 52 are shown as
one terminal or gate in the drawing, respectively, but are actually
provided in the number corresponding to the bit number of the address
signal and the control signal, respectively.
In connection with the input circuit as mentioned above, the CPU 30
generates the clamp signal of a logically high level "1" when the address
signal and the control signal are in the high impedance condition.
Otherwise, the CPU outputs the clamp signal of a logically low level "0".
Thus, when the clamp signal is at a logically high level "1", the AND gates
46 and 50 are closed. Namely, without regard to the logic levels of the
address signal and the control signal from the CPU 30, the first and
second AND gates 46 and 50 output a signal of a logically low level "0",
respectively, and therefore, all bits of the address output terminal 48
and all bits of the control signal output terminal 52 are brought into a
logically low level "0".
On the other hand, when the clamp signal is at a logically low level "0",
the AND gates 46 and 50 are opened. Therefore, the address signal and the
control signal inputted to the address input terminal 40 and the control
signal input terminal 42 from the CPU 30 are respectively passed without
modification through the AND gates 46 and 50 to the address output
terminal 48 and the control signal terminal 52. In the above mentioned
signal propagation, since there is no pull-up circuit in a signal path for
the address signal and the control signal, these signals are propagated
with a minimum delay.
Referring in FIG. 4, there is shown a circuit diagram of the AND gate used
in the input circuit of the cache controlled shown in FIG. 3. Since the
AND gates 46 and 50 are formed of AND circuits having the same
construction, one AND circuit constituting the AND gate 46 will be
explained with reference to FIG. 4. The shown AND circuit has a first
input 54 connected to receive one bit of the address signal supplied to
the address input terminal 40 and a second input 56 connected to receive
an inverted signal of the clamp signal supplied to the clamp signal input
terminal 44.
The first input 54 and the second input 56 are connected to respective
gates of a pair of p-channel transistors 58 and 60 having commonly
connected sources, which are in turn connected to a supply voltage Vdd.
These p-channel transistors 58 and 60 are commonly connected at their
drains, which are in turn connected through a pair of series-connected
n-channel transistors 62 and 64 to ground. These n-channel transistors 62
and 64 are connected at their gates to the first and second inputs 54 and
56, respectively. In addition, a connection node between the commonly
connected drains of the p-channel transistors 58 and 60 and a drain of the
n-channel transistor 62 is connected to an input of an inverter 66, whose
output is connected to an output terminal 68 corresponding to one bit of
the address output terminal 48.
When a signal inputted to the second input 56 is of O V indicative of the
logically low level, the n-channel transistor 64 is turned off, and on the
other hand, p-channel transistor 60 is turned on. At this time, if the
signal inputted to the first input 54 is at an intermediate level which
might result from the high impedance condition of the address signal
outputted from the CPU 30, the transistors 58 and 62 are put in an
unsaturated condition. However, since the transistors 60 and 64 are on and
off, respectively, a potential at the drain of the transistor 58 is put at
5 V of the supply voltage which indicates the logically high level.
Therefore, the logically high level signal is inputted to the inverter 66,
so that a logically low level signal is outputted to the output terminal
68. Thus, when the signal inputted to the second input 56 is at the low
level, the output terminal 68 is maintained at the logically low level
even if the signal inputted to the first terminal 54 is at an intermediate
level.
On the other hand, when a signal inputted to the second input 56 is for
example of 5 V indicative of the logically high level, the n-channel
transistor 64 is turned on, and the p-channel transistor 60 is turned off.
At this time, therefore, if the signal inputted to the first input 54 is
at 5 V indicative of the logically high level, the transistors 58 is
turned off and the transistor 62 are turned on so that the inverter 66
outputs the logically high level signal to the output terminal 68. If the
signal inputted to the first input 54 is at 0 V indicative of the
logically low level, the transistors 58 is turned on and the transistor 62
are turned off so that the inverter 66 outputs the logically low level
signal to the output terminal 68.
As seen from the above, if the clamp signal is supplied to the second
terminal 56, the output terminal 68 is locked to the logically low level
even if the address signal is at an intermediate level which might be
caused by the high impedance condition of the address signal outputted
from the CPU 30.
Referring to FIG. 5 there is shown a block diagram of a second embodiment
of the input circuit of the cache controller embodying the present
invention. In FIG. 5, elements similar to those shown in FIG. 3 are given
the same Reference Numerals, and explanation thereof will not be repeated.
As seen from comparison between FIGS. 3 and 5, the input circuit of the
cache controller shown in FIG. 5 includes three clamp signal terminals
44A, 44B and 44C which are connected to a logic circuit 70 for generating
a resultant clamp signal at a high speed in compliance with the system
construction. The resultant clamp signal generated by the logic circuit 70
are coupled to the respective inverted inputs of the AND gates 46 and 50.
The operation of the FIG. 5 input circuit in response to the resultant
clamp signal generated by the logic circuit 70 is the same as that of the
FIG. 3 input circuit, and therefore, explanation of the operation will be
omitted.
The logic circuit 70 receives the different signals through the clamp
terminals 44A, 44B and 44C and executes a required logic operation on
these received signals for generating the resultant clamp signal. The
logical operation of the logic circuit 70 is determined dependently upon
the system structure of the CPU 30. In the simplest example, the logic
circuit 70 is of the OR gate so that the resultant clamp signal is
generated if at least one of the signals inputted to the three clamp
signal terminals 44A, 44B and 44C is brought into a high level. Provision
of a plurality of clamp signal input terminals will make it possible to
use different signals as the clamp signal.
As will be understood from the above description, the clamp signal defines
the timing at which the address signal and/or the control signal are
clamped to the predetermined logic level. Accordingly, in the case that a
plurality of clamp signals are obtained from the CPU and a resultant clamp
signal can be generated by executing a desired logical operation on these
clamp signals, an optional selection of the type of the logic operation
would make it possible to minimize the delay from the moment a clamp
signal is inputted to one clamp input terminal to the moment the resultant
clamp signal is actually generated. Therefore, it is possible to elevate
the flexibility of the system including different types of cache memories.
Referring to FIG. 6, there is shown a block diagram of a second embodiment
of the cache controller embodying the present invention, in which a clamp
signal is formed from a HOLD request signal outputted by a DMA controller
to a CPU and status signals outputted by the CPU.
The shown system includes a CPU 72 and a DMA controller 74 which are
coupled to a data bus 76, an address bus 78 and a control bus 80,
respectively. The DMA controller 74 generates a HOLD request signal to the
CPU 72, and the CPU 72 outputs a HOLD acknowledge signal to the DMA
controller 74. Further, the address bus 78 is coupled to a cache
controller 82, which also receives CPU status signals which are extracted
from the control signals outputted by the CPU 72 onto the control bus 80.
The cache controller 82 outputs control signals to a cache data memory
coupled to the data bus 76. This cache controller 82 includes the input
circuit shown in FIG. 3, for example.
The shown system also includes a clamp signal generator 86 which is coupled
to receive the HOLD request signal from the DMA controller 74 and the CPU
status signals on the control bus 80. The clamp signal generator 86
generates a clamp signal to the cache controller 82.
In the above mentioned system, when the DMA controller 74 receives a DMA
data transfer request from a peripheral unit such as a disc system and
likes, the DMA controller 74 generates an active HOLD request signal to
the CPU 72. At this time, for example, if the CPU does not execute a
read/modify/write access operation to a main memory (not shown) (this can
be known from the CPU status signals), the clamp signal generator 86 then
generates a clamp signal in response to the active HOLD request signal.
Thus, all the address input and the CPU status signal input to the cache
controller 82 are rendered to a logically low level by the action of the
input circuit shown in FIG. 3.
After the CPU 72 has completed the bus cycle which had been in the way of
the execution when the CPU had received the active HOLD request signal,
the CPU 72 brings their address output terminal, status signal output
terminal and data input/output terminal into a high impedance condition,
and then outputs an active HOLD acknowledge signal to the DMA controller
74. As a result, the DMA controller 74 executes a data transfer between
the main memory and one peripheral unit through no intermediary of the CPU
72.
On the other hand, when the DMA controller 74 generates the active HOLD
request signal, if the CPU is in the way of the execution of the
read/modify/write access operation to the main memory, the clamp signal
generator 86 does not generate the active clamp signal until the bus cycle
is brought into a write cycle. After the bus cycle has been brought into
the write cycle (this can been known by the CPU status signals), the clamp
signal generator 86 generates the active clamp signal. On the other hand,
similarly to the above mentioned case, the CPU brings their address output
terminal, status signal output terminal and data input/output terminal
into a high impedance condition, and then outputs an active HOLD
acknowledge signal to the DMA controller 74.
As seen from the above description, the cache controller in accordance with
the present invention will clamp the address signal and/or the control
signal to a predetermined logic level when the address signal and/or the
control signal outputted from the CPU assumes a high impedance condition
without using the conventional pull-up circuit. Therefore, the delay of
the address signal and/or the control signal can be minimized.
The invention has thus shown and described with reference to the specific
embodiments. However, it should be noted that the present invention is in
no way limited to the details of the illustrated structures but changes
and modifications may be made within the scope of the appended claims.
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